The Portfolio Size Effect And Using A Bond Tent To Navigate The Retirement Danger Zone

Executive Summary

The final decade leading up to retirement, and the first decade of retirement itself, form a retirement danger zone, where the size of ongoing contributions and the benefits of continuing to work are dwarfed by the returns of the portfolio itself. As a result of this “portfolio size effect”, the portfolio becomes almost entirely dependent on getting a favorable sequence of returns to carry through.

And because the consequences of a bear market can be so severe when the portfolio’s value is at its peak, it becomes necessary to dampen down the volatility of the portfolio to navigate the danger – a strategy commonly implemented by many lifecycle and target date funds, which use a decreasing equity glidepath that drifts equity exposure lower each year.

Yet the reality is that the retirement danger zone is still limited – after the first decade, good returns will have already carried the retiree past the point of danger, and bad returns at least mean that good returns are likely coming soon, as valuation normalizes and the market cycle takes over. Which means while it’s necessary to be conservative to defend against the portfolio size effect, it’s not necessary to reduce equity exposure indefinitely.

Instead, the optimal glidepath for asset allocation appears to be a V-shaped equity exposure, that starts out high in the early working years, gets lower as retirement approaches, and then rebuilds again through the first half of retirement. Or viewed another way, the prospective retiree builds a reserve of bonds in the final decade leading up to retirement, and then spends down that bond reserve in the early years of retirement itself (allowing equity exposure to return to normal).

Ultimately, further research is necessary to determine the exact ideal shape of this “bond tent” (named for the shape of the bond allocation as it rises leading up to retirement and then falls thereafter). But the point remains that perhaps the best way to manage sequence of return risk in the years leading up to retirement and thereafter is simply to build up and then use a reserve of bonds to weather the storm.

For instance, saving $300/month allows an account balance to grow to $3,600 by the end of the first year. In the second year, the account may grow slightly, but the increase in the account balance will again be driven primarily by the contributions (as a year’s worth of growth may still be less than a single month’s worth of contributions). After 10 years of the same behavior, though, suddenly only half of the annual increase in the account balance is driven by new contributions, while the remainder is driven by growth on the existing balance. After 20 years, growth will drive 75% of the annual increases in the account balance. After 30 years, it’s almost 90%.

This phenomenon is known as the “portfolio size effect” – the mathematical recognition that in dollar terms, the impact of a portfolio’s returns is dependent on the portfolio’s size. And while this mathematical truth may seem self-evident, it has significant implications for the accumulation and decumulation of retirement portfolios. Because the portfolio size effect means that not only does growth produce a greater dollar amount of gains on a larger portfolio, but a market decline also produces a larger dollar amount of losses.

Simply put, the portfolio size effect leads to a substantial “retirement red zone” of danger that covers the final decade leading up to retirement, and the first half of retirement itself, where the portfolio’s value is so large that a potential market decline can have a catastrophic impact (given a fixed standard of living that the portfolio is intended to support).

The double-edged sword nature of the portfolio size effect – that it makes good returns even better (in dollar terms), but market declines more adversely impactful – raises challenging questions about the optimal asset allocation glidepath through the accumulation and decumulation phase.

All of which means the optimal equity exposure to manage the risks associated with the portfolio size effect over an individual’s full lifecycle would take on a V-shaped glidepath, getting more conservative in the decade leading up to retirement, remaining conservative in early retirement, and then drifting at least somewhat higher again in the later years.

Or viewed another way, if the portfolio size effect reflects when the portfolio (and the goals it is intended to support) is at the greatest risk for a catastrophe, the way to manage the danger is simply to take the least risk with the portfolio when it is the largest!

Using A Bond Tent To Dampen The Volatility Of The Portfolio Size Effect

From the traditional equity-centric perspective of portfolio management, using a V-shaped asset allocation glidepath may seem counter-intuitive, particularly when it comes to having a more conservative portfolio in the early retirement stage and then allowing it to become more aggressive again later. However, when viewed from the perspective of the portfolio’s bond allocation, the strategy appears far more logical.

After all, the fundamental purpose of bonds in a traditional portfolio is to reduce the portfolio’s volatility, which means a larger portfolio would be at less risk for a substantial financial loss. Bonds can achieve this outcome both by being an outright volatility dampener (since bonds are less volatile than stocks, swapping stocks for bonds reduces the portfolio’s overall volatility), and also as a diversifier (since stocks and bonds are not correlated, total portfolio volatility may decrease even further). Notably, in this context, the point of the bonds is not to drive returns, but to manage retirement risks and exposure to equity bear markets (which is why they’re appropriate to own, even in a low-yield environment).

Accordingly, from the bond perspective, the V-shaped glidepath turns upside down, and the prospective retiree actually accumulates extra bonds in the years leading up to retirement, and then spends down that volatility-dampening bond reserve in the first half of retirement. By the end, the retiree will finish with a bond allocation that is higher than it was in the early accumulation years, but less than it was when the portfolio was at its peak value (and therefore peak portfolio size effect risk).

In essence, the strategy to protect against the retirement danger zone and the risks that come with the portfolio size effect is to build a “bond tent” – an upside-down V-shaped extra allocation to bonds that gets built up in the final years before retirement, and gets spend down in the early years of retirement. This allows the portfolio to take shelter in the tent during the riskiest years of being exposed to the portfolio size effect – not because bonds have an appealing return, but simply because they reduce the volatility risk that becomes so severe at the portfolio’s maximal size.

Notably, there’s still far more research to be done to optimize the exact shape and the slope of the V-shaped equity glidepath and the bond tent. It’s not entirely clear how quickly during the pre-retirement red zone the bond allocation should build (i.e., the pre-retirement glidepath), nor how quickly it should be liquidated in the early retirement years. It may be that the equity exposure should be shaped more like the letter U than a V, such that the bond tent would have a wider roof – an extended period of time where greater bond allocations are held as a reserve. And the exact height of the bond tent – how high the bond allocation should reach – may be further optimized as well, especially given today’s low-yield environment (where bonds are less appealing to hold relative to historical standards, but still better than holding equities with even greater volatility and sequence risk). And of course, there are other fixed income alternatives besides traditional bonds that might be considered as “volatility dampeners” and “diversifiers” as well.

So what do you think? Would you consider a V-shaped equity glidepath in the years immediately before and after retirement? Does the idea of a bond tent – building a reserve of bonds during the years of greatest risk – make sense as a retirement strategy? Please share your thoughts in the comments below!